During loading of hydrocarbons, hydrocarbon by-products or other volatile or hazardous fluids from onsite storage to a transport tank, there is always a displacement of the gaseous environment from inside the transport tank as it is getting filled. In the case of volatile fluids, there is also off-gassing, or the release of vapours off of the volatile fluid.
As fluids are loaded, these gaseous vapors, be they displaced vapours or off-gassing vapours, are typically released from the vent system on the transport tank. Typically, in industry practice, where such gas is “sweet” (for example, containing less than 5 ppm hydrogen sulfide) venting to atmosphere has been permitted by industry and regulatory agencies. In the case of systems with “sour” gas (for example, containing greater than 5 ppm hydrogen sulfide), vented gas vapors have been traditionally directed to sweetening or scrubbing units.
With the industry norms as described above there are inherent dangers and issues:
1. During loading, explosive, flammable and hazardous gases are released into the atmosphere from the transportation tank or scrubber vent;
2. The gases being released in loading can create a localized oxygen deficient atmosphere, which is a hazardous condition for workers onsite;
3. Odour complaints from adjacent land owners;
While tank gases could be flared to deal with the above issues, transportation tanks arriving on site with oxygen present cannot be safely connected to a flare or vapour recovery unit due to potential explosive conditions.
Furthermore, in the case of on-site sweetening units, there can be hazards related to the disposal of the rich sweetening chemicals once spent. There are further concerns with hazardous, hydrogen sulphide atmospheres being created and/or production outages when the sweetening chemicals are spent at inopportune times, or prematurely.
For the purposes of the present invention transport tanks are intended to include any mobile tank systems such as, for example, tank trucks, tank trailers or rail cars or pressurized floating containers.
CA Patent Appl. No. 2,349,349 is directed to a method and apparatus for evacuating a section of a natural gas pipeline, but does not address fluids loading or unloading and the need for dealing with gas emissions during such processes. U.S. Pat. No. 8,480,812 teaches a process for removing hydrocarbon contaminants and noxious gases from catalytic reactors in the vapour phase without using steam. US Patent Application No. 2010/0000252 is directed to a process for the loading, processing and conditioning of raw production gas, the production of compressed gas liquids and the storage, transport and delivery of pipeline quality gas and other products to market. However, it does not relate to a system for transport tank loading or unloading. U.S. Pat. No. 6,901,941 is directed to a vessel for the storage and transportation of bulk volumes of fluid and methods for using the same. U.S. Pat. No. 7,252,700 is directed to a method and mobile system for cleaning dirty gas from a newly stimulated gas well.
There is a need therefore to develop improved safe, loading systems and operational methods that can deal with vapour emissions from transport tanks.